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翻译的文章Runtime.exec()

发表于2008/9/28 9:41:00 1805人阅读

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When Runtime.exec() won't

Navigate yourself around pitfalls related to the Runtime.exec() method

By Michael C. Daconta, JavaWorld.com, 12/29/00

As part of the Java language, the java.lang package is implicitly imported into every Java program. This package's pitfalls surface often, affecting most programmers. This month, I'll discuss the traps lurking in the Runtime.exec() method.

Pitfall 4: When Runtime.exec() won't

The class java.lang.Runtime features a static method called getRuntime(), which retrieves the current Java Runtime Environment. That is the only way to obtain a reference to the Runtime object. With that reference, you can run external programs by invoking the Runtime class's exec() method. Developers often call this method to launch a browser for displaying a help page in HTML.

For each of these methods, a command -- and possibly a set of arguments -- is passed to an operating-system-specific function call. This subsequently creates an operating-system-specific process (a running program) with a reference to a Process class returned to the Java VM. The Process class is an abstract class, because a specific subclass of Process exists for each operating system.

You can pass three possible input parameters into these methods:

1.A single string that represents both the program to execute and any arguments to that program

2.An array of strings that separate the program from its arguments

3.An array of environment variables

Pass in the environment variables in the form name=value. If you use the version of exec() with a single string for both the program and its arguments, note that the string is parsed using white space as the delimiter via the StringTokenizer class.

调用这些方法后，将会返回一个操作系统级的进程给JAVA VM里的Process类。你可以传三种参数给这些方法

1、带命令行参数的执行程序做为一个参数传给exec方法

2、将外部执行程序的命令和参数数组传给exec方法

3、环境变量数组传给exec方法

传环境变量的格式是name=value。如果用一个参数的exec方法，要注意命令和参数以空格分隔

Stumbling into an IllegalThreadStateException

The first pitfall relating to Runtime.exec() is the IllegalThreadStateException. The prevalent first test of an API is to code its most obvious methods. For example, to execute a process that is external to the Java VM, we use the exec() method. To see the value that the external process returns, we use the exitValue() method on the Process class. In our first example, we will attempt to execute the Java compiler (javac.exe):

If an external process has not yet completed, the exitValue() method will throw an IllegalThreadStateException; that's why this program failed. While the documentation states this fact, why can't this method wait until it can give a valid answer?

A more thorough look at the methods available in the Process class reveals a waitFor() method that does precisely that. In fact, waitFor() also returns the exit value, which means that you would not use exitValue() and waitFor() in conjunction with each other, but rather would choose one or the other. The only possible time you would use exitValue() instead of waitFor() would be when you don't want your program to block waiting on an external process that may never complete. Instead of using the waitFor() method, I would prefer passing a boolean parameter called waitFor into the exitValue() method to determine whether or not the current thread should wait. A boolean would be more beneficial because exitValue() is a more appropriate name for this method, and it isn't necessary for two methods to perform the same function under different conditions. Such simple condition discrimination is the domain of an input parameter.

Therefore, to avoid this trap, either catch the IllegalThreadStateException or wait for the process to complete.

Now, let's fix the problem in Listing 4.1 and wait for the process to complete. In Listing 4.2, the program again attempts to execute javac.exe and then waits for the external process to complete:

下面的代码改进了Listing4.1以等待外部进程结束

Listing 4.2 BadExecJavac2.java

import java.util.*;

import java.io.*;

public class BadExecJavac2

{

public static void main(String args[])

{

try

{

Runtime rt = Runtime.getRuntime();

Process proc = rt.exec("javac");

int exitVal = proc.waitFor();

System.out.println("Process exitValue: " + exitVal);

} catch (Throwable t)

{

t.printStackTrace();

}

}

}

Unfortunately, a run of BadExecJavac2 produces no output. The program hangs and never completes. Why does the javac process never complete?

不幸的是，程序挂起不能结束了，为什么会这样？

Why Runtime.exec() hangs

The JDK's Javadoc documentation provides the answer to this question:

JDK's Javadoc documentation这样说的：

Because some native platforms only provide limited buffer size for standard input and output streams, failure to promptly write the input stream or read the output stream of the subprocess may cause the subprocess to block, and even deadlock.

Is this just a case of programmers not reading the documentation, as implied in the oft-quoted advice: read the fine manual (RTFM)? The answer is partially yes. In this case, reading the Javadoc would get you halfway there; it explains that you need to handle the streams to your external process, but it does not tell you how.

JDK文档中告诉你需要处理外部进程流，但是没告诉你怎么做

Another variable is at play here, as is evident by the large number of programmer questions and misconceptions concerning this API in the newsgroups: though Runtime.exec() and the Process APIs seem extremely simple, that simplicity is deceiving because the simple, or obvious, use of the API is prone to error. The lesson here for the API designer is to reserve simple APIs for simple operations. Operations prone to complexities and platform-specific dependencies should reflect the domain accurately. It is possible for an abstraction to be carried too far. The JConfig library provides an example of a more complete API to handle file and process operations (see Resources below for more information).

Now, let's follow the JDK documentation and handle the output of the javac process. When you run javac without any arguments, it produces a set of usage statements that describe how to run the program and the meaning of all the available program options. Knowing that this is going to the stderr stream, you can easily write a program to exhaust that stream before waiting for the process to exit. Listing 4.3 completes that task. While this approach will work, it is not a good general solution. Thus, Listing 4.3's program is named MediocreExecJavac; it provides only a mediocre solution. A better solution would empty both the standard error stream and the standard output stream. And the best solution would empty these streams simultaneously (I'll demonstrate that later).

So, MediocreExecJavac works and produces an exit value of 2. Normally, an exit value of 0 indicates success; any nonzero value indicates an error. The meaning of these exit values depends on the particular operating system. A Win32 error with a value of 2 is a "file not found" error. That makes sense, since javac expects us to follow the program with the source code file to compile.

Thus, to circumvent the second pitfall -- hanging forever in Runtime.exec() -- if the program you launch produces output or expects input, ensure that you process the input and output streams.

Under the Windows operating system, many new programmers stumble upon Runtime.exec() when trying to use it for nonexecutable commands like dir and copy. Subsequently, they run into Runtime.exec()'s third pitfall. Listing 4.4 demonstrates exactly that:

As stated earlier, the error value of 2 means "file not found," which, in this case, means that the executable named dir.exe could not be found. That's because the directory command is part of the Windows command interpreter and not a separate executable. To run the Windows command interpreter, execute either command.com or cmd.exe, depending on the Windows operating system you use. Listing 4.5 runs a copy of the Windows command interpreter and then executes the user-supplied command (e.g., dir).

Running GoodWindowsExec with any associated document type will launch the application associated with that document type. For example, to launch Microsoft Word to display a Word document (i.e., one with a .doc extension), type:

>java GoodWindowsExec "yourdoc.doc"

Notice that GoodWindowsExec uses the os.name system property to determine which Windows operating system you are running -- and thus determine the appropriate command interpreter. After executing the command interpreter, handle the standard error and standard input streams with the StreamGobbler class. StreamGobbler empties any stream passed into it in a separate thread. The class uses a simple String type to denote the stream it empties when it prints the line just read to the console.

Thus, to avoid the third pitfall related to Runtime.exec(), do not assume that a command is an executable program; know whether you are executing a standalone executable or an interpreted command. At the end of this section, I will demonstrate a simple command-line tool that will help you with that analysis.

如何判断执行的外部命令是可执行程序还是命令解释程序？下面会提到一个工具来判断

It is important to note that the method used to obtain a process's output stream is called getInputStream(). The thing to remember is that the API sees things from the perspective of the Java program and not the external process. Therefore, the external program's output is the Java program's input. And that logic carries over to the external program's input stream, which is an output stream to the Java program.

One final pitfall to cover with Runtime.exec() is mistakenly assuming that exec() accepts any String that your command line (or shell) accepts. Runtime.exec() is much more limited and not cross-platform. This pitfall is caused by users attempting to use the exec() method to accept a single String as a command line would. The confusion may be due to the fact that command is the parameter name for the exec() method. Thus, the programmer incorrectly associates the parameter command with anything that he or she can type on a command line, instead of associating it with a single program and its arguments. In listing 4.6 below, a user tries to execute a command and redirect its output in one call to exec():

The program BadWinRedirect attempted to redirect the output of an echo program's simple Java version into the file test.txt. However, we find that the file test.txt does not exist. The jecho program simply takes its command-line arguments and writes them to the standard output stream. (You will find the source for jecho in the source code available for download in Resources.) In Listing 4.6, the user assumed that you could redirect standard output into a file just as you could on a DOS command line. Nevertheless, you do not redirect the output through this approach. The incorrect assumption here is that the exec() method acts like a shell interpreter; it does not. Instead, exec() executes a single executable (a program or script). If you want to process the stream to either redirect it or pipe it into another program, you must do so programmatically, using the java.io package. Listing 4.7 properly redirects the standard output stream of the jecho process into a file.

After running GoodWinRedirect, test.txt does exist. The solution to the pitfall was to simply control the redirection by handling the external process's standard output stream separately from the Runtime.exec() method. We create a separate OutputStream, read in the filename to which we redirect the output, open the file, and write the output that we receive from the spawned process's standard output to the file. Listing 4.7 completes that task by adding a new constructor to our StreamGobbler class. The new constructor takes three arguments: the input stream to gobble, the type String that labels the stream we are gobbling, and the output stream to which we redirect the input. This new version of StreamGobbler does not break any of the code in which it was previously used, as we have not changed the existing public API -- we only extended it.

Since the argument to Runtime.exec() is dependent on the operating system, the proper commands to use will vary from one OS to another. So, before finalizing arguments to Runtime.exec() and writing the code, quickly test the arguments. Listing 4.8 is a simple command-line utility that allows you to do just that.

Here's a useful exercise: try to modify TestExec to redirect the standard input or standard output to a file. When executing the javac compiler on Windows 95 or Windows 98, that would solve the problem of error messages scrolling off the top of the limited command-line buffer.

Our first test failed with an error of 193. The Win32 error for value 193 is "not a valid Win32 application." This error tells us that no path to an associated application (e.g., Netscape) exists, and that the process cannot run an HTML file without an associated application.

Therefore, we try the test again, this time giving it a full path to Netscape. (Alternately, we could add Netscape to our PATH environment variable.) A second run of TestExec produces:

This worked! The Netscape browser launches, and it then loads the Java help documentation.

One additional improvement to TestExec would include a command-line switch to accept input from standard input. You would then use the Process.getOutputStream() method to pass the input to the spawned external program.

To sum up, follow these rules of thumb to avoid the pitfalls in Runtime.exec():

总结：

1、外部程序未退出前，你无法得到外部程序返回的退出状态值

2、你必须处理外部程序的输入、输出和错误流

3、必须使用Runtime.exec()执行程序

4、不能使用Runtime.exec()执行一个像一个命令行的命令(You cannot use Runtime.exec() like a command line)

1.You cannot obtain an exit status from an external process until it has exited